Remove v3 onion service docs

[eloquence]

#208 removed references to v2 onion services, but we still have a dedicated section focused on configuring/enabling v3 onion services:

https://docs.securedrop.org/en/latest/v3_services.html

This section can be safely removed, but for orgs who have missed the Ubuntu 20.04 migration deadline, we will need to integrate the information about migrating workstation access to v3 into the migration guide if we do so.

[gonzalo-bulnes]

I suppose that whether or not some organizations are still using Ubuntu 16.04 and v2 onion services is probably sensitive. However, when reviewing the docs over the past two days (#236) I wondered if the deprecation warning and special notices should be removed. Whenever that's the case I'm happy to do it, or provide some context and list the changes as a "good first issue" for someone else to pick.